The present invention relates to gerotor devices, such as pumps and motors, and more particularly, to improved drive mechanisms for use in such devices.
Although the drive mechanism of the present invention may be utilized with various types of gerotor devices, it is especially advantageous when applied to a gerotor pump of the "fixed-axis" type, and will be described in connection therewith. In a fixed-axis gerotor pump, the gerotor includes an outer rotor, which rotates about its axis of rotation, and eccentrically disposed within the outer rotor is an inner rotor which rotates about its axis of rotation. Typically, the outer rotor is rotatably disposed within a stationary housing member.
Gerotor gear pumps of the fixed-axis type have become especially popular for use in automotive fuel pumps, which are immersed in the vehicle fuel tank and pump fuel from the tank through the fuel line to the engine. A typical automotive fuel pump includes an electric motor and a gerotor gear pump, disposed in a common housing, with the electric motor driving the gear set by means of the motor output shaft.
In a gerotor fuel pump, in which the inner rotor of the gerotor gear set is driven by an electric motor, the vehicle manufacturer specifies a certain, maximum electrical amperage (current) which is available to drive the electric motor of the fuel pump. The amperage required to drive the pump is determined by the torque needed to rotate the inner rotor of the gerotor gear set.
Ideally, the torque required to turn the inner rotor of the gerotor is quite small, being a function primarily of the output pressure of the fuel being pumped. In a typical automotive fuel pump, the output pressure is in the range of about 50-80 psi. However, in actual production, the ideal may not be achieved.
There may not be perfect concentricity amongst the axes of rotation of the motor output shaft, the inner rotor of the gerotor, the outer rotor of the gerotor, and the pumping chamber in which the outer rotor is located. It has been found that in many production gerotor fuel pumps, the deviation from perfect concentricity can be as much as 0.010 to 0.020 inches.
In typical prior art gerotor fuel pumps, the drive mechanism connecting the motor output shaft to the inner rotor of the gerotor has been relatively "fixed" or "rigid", i.e., very little relative movement is permitted between the axis of rotation of the motor output shaft and the axis of rotation of the gerotor inner rotor. As will be appreciated by those skilled in the art, when there is substantial deviation from perfect concentricity in a gerotor fuel pump utilizing a relatively rigid drive connection, one result will be a tendency for "binding" between the inner and outer rotors, thus substantially increasing the torque required to drive the inner rotor, and the amperage required by the electric motor.